The output power and resultant geographic coverage of radio and television broadcast transmission systems are regulated in the United States by the Federal Communications Commission (“FCC”). Title 47, Part 73.644 of the FCC rules and regulations regarding broadcast transmission states in part: “If electrical devices are used to determine the output power, such devices must permit determination of this power within an accuracy of ±5% of the power indicated by the full scale reading of the electrical indicating instrument of the device.”
While in-line power measurement instruments are designed and manufactured such that they are capable of measuring transmission power to within ±5% at the time of shipment, all test instruments require periodic calibration in order to maintain their design performance levels and remain in compliance with FCC rules and regulations.
Calibration approaches for currently available power measurement instruments used in broadcast applications call for the removal of the power monitor from the transmission line so that it may be returned to the factory for calibration. The major issue associated with this process is that the transmitter must be shut down while the in-line power monitor is removed from the system and temporarily replaced with either a spare power monitor or a temporary transmission line section. Due to the inherent inconvenience associated with a transmitter shut down and equipment removal, most power monitors are either calibrated very infrequently or not calibrated altogether.
Another issue associated with the current factory calibration procedures is that most factories are not capable of calibrating power monitors at the exact power levels and frequencies they are used at in the field. Because the detectors in power monitors do not provide a uniform flatline response at every frequency and power level, factory calibrated power monitors are inherently inaccurate if not used at the factory calibrated power level and frequency. These inaccuracies coupled with a drift in the calibration over time can render monitors incapable of measuring transmission line power to within ±5%, taking them out of compliance with FCC rules and regulations.
In view of these limitations, a need exists for a power monitor that is capable of being calibrated in-line during live conditions at the exact power level and frequency where it is used.
The instrument of the present invention satisfies the needs described above and affords other features and advantages heretofore not obtainable.